1. Field of the Invention
The invention relates generally to magnetoresistive (MR) sensors, and more particularly to a system that adjusts the bias of the relative magnetizations (i.e., magnetic moment or magnetization directions or vectors) of the ferromagnetic layers in the sensors.
2. Background of the Invention
One type of conventional MR sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu), silver (Ag) or a Cu or Ag alloy. One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP-MR sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the current tunneling perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as MgO, TiO2, or Al2O3.
A type of CPP sensor has been proposed that does not have a ferromagnetic reference layer with a fixed or pinned magnetization direction, but instead has dual ferromagnetic sensing or free layers separated by a nonmagnetic spacer layer. In the absence of an applied magnetic field, the magnetizations of the two free layers are oriented generally orthogonal to one another with parallel magnetization components in the sensing direction of the magnetic field to be detected and antiparallel components in the orthogonal direction. With a sense current applied perpendicularly to the layers in the sensor stack and in the presence of an applied magnetic field in the sensing direction, the two magnetizations rotate in opposite directions, changing their angle relative to one another, which is detectable as a change in electrical resistance. Because of this type of behavior of the magnetizations of the two free layers, this type of CPP sensor is often referred to as a “scissoring-type” of CPP sensor. If a CPP-GMR scissoring-type sensor is desired the nonmagnetic spacer layer is an electrically conducting metal or metal alloy. If a CPP-TMR scissoring-type sensor is desired the spacer layer is an electrically insulating material. In a scissoring-type CPP-MR sensor, a “hard-bias” layer of ferromagnetic material located at the back edge of the sensor (opposite the air-bearing surface) applies an approximately fixed, transverse magnetic “bias” field to the sensor. Its purpose is to bias the magnetizations of the two free layers so that they are approximately orthogonal to one another in the quiescent state, i.e., in the absence of an applied magnetic field. Without the hard bias layer, the magnetization directions of the two free layers would tend to be oriented antiparallel to one another. This tendency to be oriented antiparallel results from strong magnetostatic interaction between the two free layers once they have been patterned to sensor dimensions, but may also be the result of exchange coupling between the magnetic layers through the spacer layer. A scissoring-type of CPP-MR sensor is described in U.S. Pat. No. 7,035,062 B2; U.S. Pat. No. 8,670,217 B1 and U.S. Pat. No. 8,015,694 B2.
In both conventional and scissoring-type MR sensors, as the sensor size decreases with the demand for increased data density, the effect of thermomagnetic noise increases, which increases the soft error rate (SER) in the readback data signal. It is thus necessary that the sensors be manufactured with precise dimensions and magnetic properties. However, it is difficult to manufacture large volumes of sensors with identical dimensions and magnetic properties, which means that there may be a wide variation in SER among the sensors.
What is needed is a system that can adjust the bias of the ferromagnetic layers' magnetizations relative to one another after the sensors have been manufactured and installed in the disk drives, so that each sensor will have an acceptable SER.